The present invention relates to a servo system used to control movement of a flexible structure, such as stabilizing a roller driven by a motor via a gear box or stabilizing one or more axis of a gimbaled platform. More particularly, the present invention relates to methods and systems for eliminating structural modes in a servo mechanism employed to control a flexible structure, using multiple accelerometers to collectively effectively sense the structural modes and compensating for the sensed structural modes in the servo mechanism.
Flexible structures, in general, cause problems with stability for servo mechanisms or controllers. Typically, such a flexible structure is comprised of at least two masses and at least one spring like element where each spring element couples every two masses. Each spring element functions as a spring when the flexible structure is subject to a linear or rotational input or disturbance at or above a particular disturbance frequency (e.g., the frequency at which the spring element exhibits flexure). The effect of the spring, however, may be a gradual one until the resonant frequency is reached. Typically, the effect of the spring below the resonant frequency is quite small. In one example of a flexible structure, a motor attached to and driving a shaft to rotate or linearly adjust the position of a mirror attached to the shaft may be collectively characterized as a flexible structure. In this example, the motor and the mirror function as respective rigid masses while the shaft may function as a torsion spring at a particular drive frequency and contribute to a resonance of this flexible structure. Another example of a flexible element is a motor that is attached to a gearbox that is attached to a roller for a press, rolling mill, or other machine. In this example, the motor and the roller function as the two masses while the gear box function as a torsion spring at a particular drive frequency. Likewise, in a gimbal system, a torquer motor attached to a shaft for driving a gimbal attached to a platform mount may be collectively viewed as a flexible structure. In this example, the torquer motor and gimbal may be viewed as two masses and the shaft may function as a torsion spring at a particular drive or disturbance frequency. In fact, certain components of a gimbal system (such as the yoke forming an azimuth gimbal) may appear to be continuously formed of one or more materials that act as a rigid body at low linear or rotational input frequencies (such as inputs from an the azimuth gimbal motor) but at high input frequencies act as a flexible structure with separate masses coupled by a respective torsion spring element. For example, the yoke of an azimuth gimbal may have a lower portion corresponding to a first mass, the ends of the arms of the azimuth gimbal yoke (which are typically pivotally attached to elevation gimbal) may correspond to a second mass, and the arms of the azimuth gimbal yoke below the yoke ends may function as a spring element at high drive input frequencies from the azimuth gimbal motor.
There are many papers in published trade literature and many patents that have been issued concerning the control of flexible structures. However, a search of the literature and patents does not reveal any fundamental system or method for controlling a flexible structure.
The primary measure of performance for a gimbal used to stabilize optics mounted on a platform or bed attached to the gimbal is jitter or movement of the line of sight (LOS) of the optics when the gimbal is being vibrated due to one of several causes, including the structural deformation of the flexible structure comprising the gimbal and optical bed or platform. Historically, a stabilization servo controller and a rate sensor, such as a gyro, positioned on the gimbal have been used to stabilize the gimbal and, thus, the LOS of the optics. The servo controller uses the gyro signal as a feedback signal, and, by sending a signal to a power amplifier which controls a torquer motor, attempts to keep the LOS of the optical system stable. This approach has been used for many years but does not compensate for LOS jitter caused by flexible structure deformation.
The LOS jitter for a gimbal is reduced if the zero dB crossover frequency of the stabilization servo is increased, so the objective of the servo design is to have the zero dB crossover frequency as high as possible to minimize the jitter. The desire for having the zero dB crossover frequency as high as possible is a result of the desire to have the gain as high as possible. It is gain that reduces the LOS jitter. The zero dB crossover frequency is not the same as the bandwidth, but should be close to the −90 deg phase of the closed loop servo. Five factors limit the zero dB crossover frequency: The gimbal structure, the frequency response of the rate sensor (gyro, quartz rate sensor (QRS), etc.), the iteration rate of the servo controller (assuming it is a digital controller), the bandwidth of the power amplifier driving the torquer motor, and the noise of the rate sensor). All sensors have some noise, and this noise must not saturate the power amplifier driving the torquer motor.
The principles of the present invention deal with the problem of the deformation of the structure, which may be characterized as a flexible structure as noted above. Historically, structural modes have been dealt with in several ways. The most common way is to limit the servo controller performance by having the 0 dB crossover frequency below the first structural mode at which the structure first exhibits flexure. This approach can severely limit the servo performance. Another common way is to use a notch filter and reduce the gain of the structural resonance. This allows the servo to have its 0 dB crossover closer to the structural resonance, but still limits the performance of the servo due to the phase loss of the notch filter. A third method, which is often used in aircraft and space craft autopilots, is to “phase stabilize” the structural deformation modes. However, a servo is not limited to just one crossover frequency. Multiple crossover frequencies are allowed as long as Nyquist's stability criteria are not violated. Phase stabilizing a servo has a very significant drawback in which the low frequency gain of the servo is limited by the stability requirements at the structural modes, and this limits the low frequency gain. High gain at low frequency is necessary to provide good LOS stabilization for a gimbal, good step response, low static and dynamic error, and good rejection of disturbances.
There is therefore a need for a servo system that overcomes the problems noted above and enables the realization of a method and a system for eliminating structural deformation modes in a servo mechanism or controller employed to control a flexible structure.